Method of making a composition for separating and concentrating certain ions from mixed ion solutions

ABSTRACT

A method for removing, separating, and concentrating certain selected ions from a source solution that may contain larger concentrations of other ions comprises bringing the source solution in contact with a composition comprising an ion-binding ligand covalently bonded to a membrane having hydrophilic surface properties. The ligand portion of the composition has affinity for and forms a complex with the selected ions, thereby removing them from the source solution. The selected ions are then removed from the composition through contact with a much smaller volume of a receiving solution in which the selected ions are either soluble or which has greater affinity for the selected ions than does the ligand portion of the composition, thereby quantitatively stripping the complexed ions from the ligand and recovering them in concentrated form in the receiving solution. The concentrated ions thus removed may be further separated and recovered by known methods. The process is useful in the removal of selected ions, including noble metals and other transition metals from a variety of source solutions such as are encountered in semiconductor, nuclear waste cleanup, metals refining, environmental cleanup, providing ultra high purity fluids, electric power, and other industrial enterprises. The invention is also drawn to the ligand-membrane compositions.

This application is a division of application Ser. No. 08/685,432 filedJul. 23, 1996, U.S. Pat. No. 5,618,433, which is a divisional of Ser.No. 08/233,640 filed Apr. 26, 1994, now U.S. Pat. No. 5,547,760.

FIELD OF THE INVENTION

This invention relates to ion-binding ligands covalently bonded tomembranes and to a process for removing and concentrating certainselected ions from solutions using the ligand-membrane compositions,wherein such ions may be admixed with other ions present in much higherconcentrations. More particularly, the invention relates toligand-membrane compositions and to a process for removing such ionsfrom an admixture with other ions in a source solution by forming acomplex of the selected ions with the ligand-membrane compositions byflowing such solutions through a contacting device containing theligand-membrane compositions and then breaking the complex of theselected ion from the composition to which such ion has become attachedby flowing a receiving liquid in much smaller volume than the volume ofsolution passed through the contacting device to remove and concentratethe selected ions in solution in the receiving liquid. The concentratedions thus removed may then be recovered by known methods.

BACKGROUND OF THE INVENTION

Composite membranes of the type utilized in one embodiment of thepresent invention have been previously described in U.S. Pat. No.4,618,533 to Steuck. Some of the ion-binding ligands of the typesdisclosed herein are also known. For example, U.S. Pat. No. 4,952,321 toBradshaw et al. discloses amine-containing hydrocarbons attached to asolid inorganic support such as silica or silica gel wherein the ligandis bound to the solid inorganic support through a hydrocarbon spacercontaining a S trialkoxysilane group. U.S. Pat. Nos. 5,071,819 and5,084,430 to Tarbet et al. disclose sulfur and nitrogen-containinghydrocarbons as ion-binding ligands. U.S. Pat. Nos. 4,959,153 and5,039,419 to Bradshaw et al. disclose sulfur-containing hydrocarbonligands. U.S. Pat. Nos. 4,943,375 and 5,179,213 to Bradshaw et al.disclose ion-binding crowns and cryptands as ligands. U.S. Pat. No.5,182,251 to Bruening et al. discloses aminoalkylphosphonicacid-containing hydrocarbons ligands. U.S. Pat. No. 4,960,882 toBradshaw discloses proton-ionizable macrocyclic ligands. U.S. Pat. No.5,078,978 to Tarbet et al. discloses pyridine-containing hydrocarbonligands. U.S. Pat. No. 5,244,856 to Bruening et al. disclosespolytetraalkylammonium and polytrialkylamine-containing hydrocarbonligands. U.S. Pat. No. 5,173,470 to Bruening et al. discloses thioland/or thioether-aralkyl nitrogen-containing hydrocarbon ligands. U.S.Pat. No. 5,190,661 to Bruening et al. discloses sulfur-containinghydrocarbon ligands also containing electron withdrawing groups.Copending application Ser. No. 08/058,437 filed May 7, 1993, disclosesoxygen donor macrocycles, for example, ligands containing macrocyclicpolyether cryptands, calixarenes, and spherands, multiarmed ethers andmixtures of these. All of these previous reports have involved bindingof the ligands to solid inorganic supports via a silane-containingspacer grouping. However, researchers have not previously reportedincorporating complex, strongly interacting and highly selectiveion-binding ligands into membranes which would be highly desirablebecause of the high surface-to-area ratios, convenient physical formats,ease of production, ease of use, and inexpensive cost of such membranes.The present invention successfully accomplishes this feat.

SUMMARY OF THE INVENTION

The compositions of the present invention comprise ion-binding ligandsthat are covalently bonded to a membrane through an amide, ester,thioester, carbonyl or other suitable bond. Membranes that areinherently hydrophilic, or partially hydrophilic, and contain moietiesappropriate for making these bonds are preferred. Such membranes includepolyamides, such as nylon, and cellulosic materials, such as cellulose,regenerated cellulose, cellulose acetate, and nitrocellulose. If themembrane used does not contain reactive groups it may be modified orderivatized appropriately. Composite membranes are also useful. Acomposite membrane comprises a porous polymer membrane substrate and aninsoluble, cross-linked coating deposited thereon. Representativesuitable polymers forming the membrane substrate include fluorinatedpolymers including poly(tetrafluoroethylene) ("TEFLON"), polyvinylidenefluoride (PVDF), and the like; polyolefins such as polyethylene,ultra-high high molecular weight polyethylene (UPE), polypropylene,polymethylpentene, and the like; polystyrene or substitutedpolystyrenes; polysulfones such as polysulfone, polyethersulfone, andthe like; polyesters including polyethylene terephthalate, polybutyleneterephthalate, and the like; polyacrylates and polycarbonates; and vinylpolymers such as polyvinyl chloride and polyacrylonitriles. Copolymerscan also be used for forming the polymer membrane substrate, such ascopolymers of butadiene and styrene, fluorinated ethylene-propylenecopolymer, ethylene-chlorotrifluoroethylene copolymer, and the like.

With composite membranes, the substrate membrane material is not thoughtto affect the performance of the derivatized membrane and is limited incomposition only by its ability to be coated, or have deposited on itssurface, an insoluble polymer layer that contains the appropriatereactive group. This provides a hydrophilic layer which interacts wellwith water or other aqueous solutions. The end result is that when anorganic ligand is attached to the surface of either a hydrophilicmembrane or a composite membrane having a hydrophilic surface, the basiccharacteristics of any given ligand molecule are not changed by theprocess of attaching it to the surface or by the nature of the surfaceitself.

The coating of composite membranes comprises a polymerized cross-linkedmonomer. Representative suitable polymerizable monomers includehydroxyalkyl acrylates or methacrylates including 1-hydroxyprop-2-ylacrylate and 2-hydroxyprop-1-yl acrylate, hydroxypropylmethacrylate,2,3-dihydroxypropyl acrylate, hydroxyethylacrylate, hydroxyethylmethacrylate, and the like, and mixtures thereof. Other polymerizablemonomers that can be utilized include acrylic acid,2-N,N-dimethylaminoethyl methacrylate, sulfoethylmethacrylate and thelike, acrylamides, methacrylamides, ethacrylamides, and the like. Othertypes of hydrophilic coatings that can be used within the scope of theinvention include epoxy functional groups such as glycidyl acrylate andmethacrylate, primary amines such as aminoethyl methacrylates, andbenzyl derivatives such as vinyl benzyl chloride, vinyl benzyl amine,and p-hydroxyvinyl benzene.

The coating of composite membranes also comprises a precipitated crystalsystem, such as that involving the material known under the trademark"NAFION." "NAFION" is a sulfonic acid or sodium sulfonate of aperfluorinated polyether.

The basic consideration in selecting a composite membrane is that thecoating placed on the membrane substrate is the determining factor indefining the chemistry used to covalently attach the ligand. Forexample, a composite membrane displaying a carboxylic acid functionalgroup can form an amide bond with a pendant amine group from the ligand,one of the most stable methods of ligand immobilization. The compositepolymers referenced above can be prepared with carboxylic acid activegroups that can be readily converted to amides upon reaction with anamine group on a ligand. However, any of the other organic species whichare reactive toward an acid chloride could be used to attach an organicligand to the surface. Additional examples of such groups would beesters, thioesters, Grignard reagents, and the like.

If the reactive group on the surface is a sulfonic acid, then ananalogous procedure using a sulfonyl chloride would yield resultssimilar to those obtained with carboxylic acid functionalities. One suchpolymer containing sulfonic acid reactive groups is available under thetradename "NAFION" from DuPont as described above.

The ligand is selected from the group consisting of amine-containinghydrocarbons, sulfur and nitrogen-containing hydrocarbons,sulfur-containing hydrocarbons, crowns and cryptands,aminoalkylphosphonic acid-containing hydrocarbons,polyalkylene-polyamine-polycarboxylic acid-containing hydrocarbons,proton-ionizable macrocycles, pyridine-containing hydrocarbons,polytetraalkylammonium and polytrialkylamine-containing hydrocarbons,thiol and/or thioether-aralkyl nitrogen-containing hydrocarbons,sulfur-containing hydrocarbons also containing electron withdrawinggroups, and macrocyclic polyether cryptands, wherein the ligands arecapable of selectively complexing ions such as either certain alkali,alkaline earth, noble metal, other transition metal, and post transitionmetal ions when contacted with solutions thereof when admixed with otherions.

The process for removing and concentrating certain selected ions usingthe ligand-membrane compositions is carried out in any manner thatprovides for bringing the ion to be removed into contact with the ligandaffixed to the membrane. Overall the process comprises selectivelyremoving and concentrating one or more selected species of ion from aplurality of other ions in a multiple ion solution in which the otherions may be present at much higher concentrations. The multiple ionsolution or source solution is brought into contact with a compositionof the present invention. The preferred embodiment disclosed hereininvolves carrying out the process by bringing a large volume of themultiple ion solution into contact with a composition of matter of theinvention. Contact is preferably made in a contacting device comprisinga cartridge containing the composition of matter of the invention bycausing the multiple ion solution to flow through the cartridge and thuscome in contact with the composition of the invention. However, variouscontact apparatus may be used instead of a cartridge. The selected ionor ions complex with the composition. Following the complexing step, asmall volume of a receiving liquid or eluant is brought into contactwith the loaded composition to break the complex by chemical or thermalmeans and to dissolve the selected ions and carry them away from thecomposition. The selected ions can then be recovered from the receivingliquid by well known procedures.

More particularly, the process comprises forming a completing agent bycovalent bonding of a ligand of the type mentioned previously to acomposite membrane, such as one of those previously mentioned. Thecomplexing agent is then introduced into a contacting device such as acartridge. The solution containing the multiple ion species flowsthrough the cartridge in contact with the complexing agent, whereby theselected ions complex with the complexing agent. The selected ions arethus separated from the rest of the ion mixture that flows out of thecartridge. A small volume of the receiving liquid or eluant is thenpassed through the cartridge to break the complex and dissolve and carryout of the cartridge the selected ion or ions. The selected ions arethen recovered from the receiving phase by well known procedures.

DETAILED DESCRIPTION OF THE INVENTION

Preparation of the Ligand-Membrane Compositions

The compositions of the present invention may be prepared by anysuitable method wherein the ligands can be covalently bonded to amembrane containing reactive functional groups.

The membrane is selected to yield both selected bulk properties andselected surface properties. For naturally hydrophilic membranes, theselected bulk and surface properties will be provided by whateverpolymer that comprises the membrane. For composite membranes, theselected bulk properties will be provided by the membrane substrate andthe selected surface properties will be provided by the coating. Acomposite membrane is formed by depositing a monomer directly on thesurface of the substrate, including the inner surfaces of the pores, byin situ deposition of the cross-linked monomer. The desired depositionof the cross-linked monomer onto the porous substrate is effected as adirect coating and does not require or utilize an intermediate bindingchemical moiety. Any monomer for the coating polymer can be used so longas it is capable of being polymerized by free radical polymerization andcan be cross-linked. The only requirements of the polymerized monomer isthat it is capable of coating the entire surface of the porous membrane,that it provide the surface with ligand-reactive functional groups, andthat it be sufficiently hydrophilic to allow for efficient use of theligand to be attached. Generally, the porous substrate has an averagepore size between about 0.001 and 10 μm, and more usually, between about0.1 and 5.0 μm. The composite membrane is formed by any suitable method,such as is disclosed in U.S. Pat. No. 4,618,533, which is herebyincorporated by reference. Briefly, this procedure involves washing theporous membrane substrate with a suitable solvent for wetting the entiresurface of the substrate. The substrate is then bathed in a mixture ofthe free radical polymerizable monomer, a polymerization initiator, anda cross-linking agent in a solvent under conditions to effect freeradical polymerization of the monomer and coating of the poroussubstrate with the cross-linked polymer. The surface of the coatedpolymer membrane contains hydrophilic or polar-substituents that can beactivated to react with and covalently bond the ligands to the membranesurface.

The composite membranes prepared according to U.S. Pat. No. 4,618,533can contain carboxylic acid moieties on the surface. Other suitablemoieties could include hydroxyl, sulfonic acid, epoxy, primary amine,and derivatized benzyl groups such as polymers referenced above.

Preparation of a composite membrane by a precipitated crystal techniqueinvolves, briefly, washing the porous membrane substrate with a suitablesolvent for wetting the entire surface of the substrate. The substrateis then bathed in a solution containing the crystals that are to beprecipitated. This solution is then removed and the membrane substrateis treated with a compound that precipitates and fixes the crystals tothe substrate. The membrane is washed and dried before use.

In the present invention, the activation of the carboxylic acid groupsis exemplified by the reaction of the carboxylic acid groups withthionyl chloride to form acid chloride groups according to the formula:

    membrane-COOH+S(O)Cl.sub.2 →membrane-C(O)Cl+SO.sub.2 +HCl

Carboxylic acid groups also can be converted to acid chloride groups byreaction with phosphorus pentachloride or phosphorus trichloride.

Ligands (L) containing reactive amines, alcohols, thiols, Grignardreagents and the like may be covalently bonded to the membrane throughthe --C(O)Cl group as follows:

(1) membrane-C(O)Cl+H₂ NL→membrane-C(O)NHL+HCl (amide)

(2) membrane-C(O)Cl+HOL→membrane-C(O)OL+HCl (ester)

(3) membrane-C(O)Cl+HSL→membrane-C(O)SL+HCl (thioester)

(4) membrane-C(O)Cl+XMgL→membrane-C(O)L+MgXCl (ketone)

In a similar manner, the activation of the sulfonic acid groups isexemplified by the reaction of the sulfonic acid groups with thionylchloride to form sulfonyl chloride groups according to the formula:

    membrane-S(O).sub.2 OH+S(O)Cl.sub.2 →membrane-S(O).sub.2 Cl+SO.sub.2 +HCl

Sulfonyl chloride groups also can be obtained by reaction of sulfonicacid groups with phosphorus pentachloride or phosphorus trichloride.

Ligands containing reactive amines, alcohols and the like may becovalently bonded to the membrane through the --S(O)₂ Cl group asfollows:

(1) membrane-S(O)₂ Cl+H₂ NL→membrane-S(O)₂ NHL+HCl (sulfonamide)

(2) membrane-S(O)₂ Cl+HOL→membrane-S(O)₂ OL+HCl (sulfonate ester)

This reaction does not proceed as readily as the reactions with acidchlorides formed from carboxylic acids. However, any reaction may beused provided it is functional to form a stable covalent bond betweenthe ligand and the membrane. For the present, it has been found that theamide linkage is most stable and readily formed.

Ligands, which may be adapted to contain --NH₂, --OH, --SH, --MgXmoieties which are reactive so as to form a covalent bond with membraneattached functionalities are illustrated in the patents indicated below,which are hereby incorporated by reference: amine-containinghydrocarbons (U.S. Pat. No. 4,952,321), sulfur and nitrogen-containinghydrocarbon ligands (U.S. Pat. Nos. 5,071,819 and 5,084,430),sulfur-containing hydrocarbon ligands (U.S. Pat. Nos. 4,959,153 and5,039,419), crowns and cryptand ligands (U.S. Pat. Nos. 4,943,375 and5,179,213), aminoalkylphosphonic acid-containing hydrocarbon ligands(U.S. Pat. No. 5,182,251), proton-ionizable macrocycle ligands (U.S.Pat. No. 4,960,882), pyridine-containing hydrocarbon ligands (U.S. Pat.No. 5,078,978), polytetraalkylammonium and polytrialkylamine-containinghydrocarbon ligands (U.S. Pat. No. 5,244,856), thiol and/orthioether-aralkyl nitrogen-containing hydrocarbon ligands (U.S. Pat. No.5,173,470), and sulfur and electron withdrawing group-containinghydrocarbon ligands (U.S. Pat. No. 5,190,661).

An oxygen donor macrocycle ligand, such as disclosed in copendingapplication Ser. No. 08/058,437 filed May 7, 1993, having a reactivegrouping attached, may be prepared by various reaction schemes. Two areillustrated. The first involves the reaction of a cis dihydroxy crownether with a polyether diol wherein the diol groups have been activatedby reaction with a "leaving" group such as tosyl chloride. The followingreaction sequence (Reaction A) shows the formation of an oxygen donormacrocycle ligand (Formula 2) by means of reacting a cis. dihydroxycrown ether (Formula 3) with a tosylated polyether diol (Formula 4) asfollows wherein Ts stand for the tosyl group, R₃, R₄, R₅, and R₆ is eacha member independently selected from the group consisting of H,allyloxymethyl, alkylthio, alkylamino, carboxy, carboxyalkyl, andepoxyalkyl. R₇ is a member selected from the group consisting of H andalkyl, Z is a member selected from the group consisting of o-phenyleneand o-naphthalene or alkyl, R₁ and R₂ are each a member selected fromthe group consisting of H, allyl, alkenyl, carboxy, carboxyalkyl,allyloxy, aminoalkyl, hydroxy, thio, and alkylthio. The functionalgroups that are not directly reactive with the corresponding groups onthe surface of the membrane must be further reacted so as to allow acovalent bond. As an example, a carboxy alkyl functional group could beconverted to an acid chloride and further reacted with ethylene diamine(in large excess) to provide a mono amide with a free amine. This couldthen be reacted with the membrane. Further, n is an integer of 2 to 4, ais an integer of 0 or 1, b is an integer of 0 to 3 with the proviso thatb must be at least 1 when a is 0, and m is an integer of 0 to 5. Toprovide a reactive grouping to react with a reactive membrane, it ismandatory that one or two, and preferably only one, of the R₁ through R₆groups must be other than H. The remaining R₁ through R₆ groups are H.##STR1##

While the Ts or tosyl group is illustrated above, other leaving groupssuch as mesylates, chlorides, bromides and the like can also beutilized. The tosyl group is preferred because it is crystalline and hasbetter reaction properties.

The second reaction scheme involves the reaction of a cis dibromomethylcrown ether with a polyether diol. The following reaction sequence(Reaction B) shows the formation of an oxygen donor macrocycle ligand(Formula 2) by means of reacting a cis dibromomethyl crown ether(Formula 5) with a polyether diol (Formula 6) as follows wherein symbolshave the same meaning as given for Formula 2 above: ##STR2##

The compound corresponding to Formula 2, having a reactive grouping maythen be reacted with a membrane derivatized with hydrophilicfunctionalities.

Polyalkylene-polyamine-polycarboxylic acid-containing hydrocarbonligands may be prepared by various methods. For example, in one methodthe polyalkylene-polyamine-polycarboxylic acid ligand is bound to themembrane. In a second method, a polyalkylenepolyamine is reacted with amembrane followed by reacting with a polycarboxylic acid.

The above described ligands have heretofore been attached to solidsupports such as silica gel, silica, glass, glass fibers, nickel oxide,zirconia, alumina, titania and the like. The attachment of the ligand tothe solid support has been by means of a silane spacer grouping. Thereare certain drawbacks to the use of such solid support. For example,they most often have to be contained in a column or similar structureand do not have the adaptability for other configurations that amembrane possesses. Further, silane chemistry is complicated and limitscertain reactions or applications. Finally, the instability or evenpartial dissolution of the inorganic supports in some solution matricesmakes their use in some separation applications poor or unacceptable.However, such ligands, that have been attached to the above mentionedinorganic solid supports, have not previously been affixed to membranes.

The novelty of the invention is in the membrane ligand combination andin the method of using such combinations in removing desired ions. Anyof the ligands previously used may be modified for use in the presentinvention. Because the ligands are not in and of themselves novel, theywill be referred to as ligands ("L") and may be further designated byclasses, i.e. amine-containing hydrocarbon ligands; sulfur andnitrogen-containing hydrocarbon ligands; sulfur-containing hydrocarbonligands; crown and cryptand ligands; aminoalkylphosphonicacid-containing hydrocarbon ligands; proton-ionizable macrocycleligands; pyridine-containing hydrocarbon ligands; polytetraalkylammoniumand polytrialkylamine-containing hydrocarbon ligands; thiol and/orthioether-aralkyl nitrogen-containing hydrocarbon ligands; sulfur andelectron withdrawing group-containing hydrocarbon ligand; and oxygendonor macrocycle ligands. This listing of ligands is exemplary only andis not intended to be all encompassing. Other ligands, known or yet tobe developed, may also be utilized with the only limitation being thatthey can be covalently attached to the membrane and are functional inthe selective attracting and binding of the selected ions being removedfrom the solutions being treated.

The membrane ligand combination of the invention can therefore bedefined by the formula:

    M-B-L

wherein M is any membrane or composite membrane derivatized to have ahydrophilic surface and contain polar functional groups, L is any ligandas defined above containing a functional grouping reactive with anactivated polar group from the membrane and B is the covalent linkageformed by the reaction between the activated polar group and thefunctional group of the ligand.

Representative of B linkages are members selected from the groupconsisting of amide (NHCO), ester (COO), thioester (COS), carbonyl (CO),ether (O), thioether (S), and sulfonamide (SO₂ NH).

The membrane/ligand compositions of the present invention that areuseful for separating selected ions will be apparent to those skilled inthe art by the following examples each of which utilizes a compositemembrane prepared according to U.S. Pat. No. 4,618,533 and containingcarboxylic acid groups or sulfonic acid groups.

EXAMPLE 1

In this example, a nitrogen-containing ligand derivatized membrane wasprepared according to the following procedure. A 3×3 inch piece ofpolytetrafluoroethylene (PTFE) ("TEFLON") membrane coated by the methodof U.S. Pat. No. 4,618,533 with crosslinked acrylic acid containingcarboxylic acid functional groups immobilized on the surface wasimmersed in enough thionyl chloride to completely cover the surface ofthe membrane. The membrane remained covered by this solution for 8-14hours to enable the thionyl chloride to react with and convert thecarboxylic acid groups to acid chlorides. The activated membrane wasthen removed and washed thoroughly with hexane. Other organic solvents,such as toluene, would work equally well. The activated membrane wasthen placed in a flask containing a solution composed of 3 g ofpentaethylenehexamine ligand and enough toluene to be sure the membranewas completely covered by the mixture. This mixture was allowed to reactfor 8-14 hours to form an amide bond between one of the amine groups ofthe ligand and the acid chloride group of the membrane. The membrane wasagain washed with organic solvent to remove unbound ligand and permittedto air dry in a well-ventilated hood. After the membrane was dried itwas tested to determine its ion binding properties. Testing results areshown in Example 14.

EXAMPLE 2

In this example, a 3×3 inch piece of polyvinylidene fluoride (PVDF)membrane coated by the method of U.S. Pat. No. 4,618,533 withcrosslinked acrylic acid containing carboxylic acid functional groupswas converted to the acid chloride form and then derivatized withpentaethylenehexamine as in Example 1.

In Examples 3-12 which follow the carboxylic acid derivatized PTFEcomposite membrane of Example 1 was utilized for ligand attachment.However, the PVDF composite membrane of Example 2 could have been usedwith similar results. When testing the separation properties of ligandsaffixed to composite membranes of both Examples 1 and 2, the resultswere substantially the same.

EXAMPLE 3

In this example, a nitrogen and sulfur-containing ligand derivatizedmembrane was prepared according to the following procedure. A 3×3 inchsquare of carboxylic acid group containing PTFE composite membrane wasprepared and treated with thionyl chloride as in Example 1. Thismaterial was then reacted with pentaethylenehexamine as a first step toattach the amine via an amide bond to the membrane. This intermediateproduct was then washed, and immersed in a second solution containingtoluene and 1 g of ethylene sulfide to provide the ligand with a --NHCH₂CH₂ SH grouping. Again, it was necessary to ensure that the solutioncovered the membrane at all times. The reaction times for each step arefrom 8-14 hours. After the membrane was dried, it was tested for ioncomplexation properties as shown in Example 15.

EXAMPLE 4

In this example, a nitrogen and sulfur-containing ligand derivatizedmembrane was prepared according to the following procedure. A 3×3 inchsquare of carboxylic acid group containing PTFE composite membrane wastreated with thionyl chloride as in Example 1. This material was thenreacted with ethylene diamine instead of pentaethylenehexamine as inExample 3. The result of this reaction is a material that is bonded tothe membrane via an amide linkage and contains one free amino group thatis then further reacted with a solution containing toluene and ethylenesulfide as in Example 3. After the membrane was dried, it was tested forion complexation properties as shown in Example 16.

EXAMPLE 5

In this example, a sulfur-containing ligand derivatized membrane wasprepared according to the following procedure. The carboxylic acid groupcontaining PTFE composite membrane was prepared as in Example 4 so thatthe carboxylic acid groups were converted to the acid chloride form. Themembrane was then immersed in a solution containing toluene and thereaction product of ethanedithiol and one equivalent of 2-methylaziridine to immobilize a --CONHCH₂ CH(CH₃)SCH₂ CH₂ SH ligand on themembrane. The free SH group was then blocked with a methanol solutioncontaining methyl iodide and sodium carbonate. After the membrane wasdried, it was tested for ion complexation properties as shown in Example17.

EXAMPLE 6

In this example, a crown ether containing ligand was prepared andattached to a membrane according to the following procedure. The acidchloride form of the carboxylic acid group containing PTFE compositemembrane was prepared as in Example 1. The crown was prepared forattachment by taking 2 g of allyloxymethyl-18-crown-6 and dissolving itin either dichloromethane or benzene. The double bond of the allyl groupwas then converted into the epoxide by adding hydrogen peroxide (1 to 2small drops of a 30% solution) to the stirring mixture. Ammoniumhydroxide (0.2 g) was then added to the epoxidized crown and thetemperature was raised to between 30° C. and 60° C. The reaction wasallowed to proceed for 6-14 hours to form a ligand comprising 18-crown-6containing a --CH₂ OCH₂ CH(OH)CH₂ NH₂ grouping. This ligand-containingreaction mixture was added to a toluene solution containing themembrane. This procedure resulted in the 18-crown-6 being attached viaan amide linkage and can also be used to attach a wide variety of othermacrocyclic compounds, or starting materials containing double bonds.After the membrane was dried, it was tested for ion complexationproperties as shown in Example 18.

EXAMPLE 7

In this example, an aminophosphonic acid-containing ligand derivatizedmembrane was prepared according to the following procedure. A 3×3 inchsquare of carboxylic acid group containing PTFE composite membrane wastreated with thionyl chloride and ethylene diamine as in Example 4. Theresulting amino-amide was further reacted by placing the membrane into a3-necked round bottom flask containing 83 ml concentrated HCl, 83 mlwater, and 70 g of phosphorous acid. The mixture was heated to reflux,and 270 ml of formaldehyde was slowly added over a period of 1 hour. Themixture was refluxed for 1 to 4 additional hours resulting in a ligandattached via an amide linkage comprising the grouping CONHCH2CH₂ N(CH₂PO(OH)₂)₂. This product was washed with water, and dried. This productwas then tested for its ion complexation properties as shown in Example19.

EXAMPLE 8

In this example, the procedure of Example 7 was followed with theexception that pentaethylenehexamine was substituted for ethylenediamine, with the volumes of reagents being adjusted in accordance withthis substitution. This results in a ligand comprising the grouping--CONH(CH₂ CH₂ NH)₅ CH₂ PO(OH)₂. This product was then tested for itsion complexation properties as shown in Example 20.

EXAMPLE 9

In this example, a nitrogen-containing ligand derivatized membrane wasprepared according to the following procedure. A 3×3 inch piece of PTFEcomposite membrane with carboxylic acid groups on the surface accordingto Example 1 was converted to the acid chloride form and reacted withtetraaza-12-crown-4 in toluene with the resultant formation of an amidebond between one of the ring nitrogen atoms and the acid chloride. Theresulting membrane was washed 4 times with toluene and then treated withconcentrated HCl, phosphorous acid, and formaldehyde as in Example 7 toproduce a membrane with a macrocyclic aminoalkylphosphonic pendentgroup. This material was then tested for ion complexing properties asshown in Example 21.

EXAMPLE 10

In this example, an aminocarboxylic acid-containing membrane wasprepared according to the following procedure. The material was preparedas in Example 7 up to the point of having ethylene diamine attached tothe surface via an amide linkage. This material was further reacted byplacing the membrane into a flask is containing 200 ml dimethylformamide(DMF), 0.1 g dimethylaminopyridine (DMAP), 25 ml pyridine, and 1 g ofdiethylenetriaminepentaacetic acid (DTPA) dianhydride. The mixture wasallowed to react at 80° C. for 24-72 hours. The final product was washedwith water, dried, and tested for ion binding properties as shown inExample 22.

EXAMPLE 11

In this example, a nitrogen-containing cryptand was attached to acarboxylic acid group containing PTFE composite membrane according tothe following procedure. The procedure for producing a membrane withcryptand 2.2.2 attached thereto was identical to the procedure used inExample 6 except that allyloxymethyl-cryptand-2.2.2 was used in place of18-crown-6. After the membrane was dried, it was tested for ioncomplexation properties as shown in Example 23.

EXAMPLE 12

In this example, a nitrogen-containing crown was attached to a membraneaccording to the following procedure. An acid chloride form of thecarboxylic acid group containing PTFE composite membrane was prepared asin Example 1. Hexaza-18-crown-6 dissolved in toluene was then allowed toreact with the membrane for 8-14 hours as in Example 9. The membrane waswashed with toluene and dried before testing the ion removal propertiesas shown in Example 24.

EXAMPLE 13

In this example, an ultra-high molecular weight polyethylene (UPE)membrane was coated with "NAFION" by a precipitated crystal technique toyield a membrane having sulfonic acid reactive groups on the surface,and then a nitrogen-containing ligand-derivatized membrane was prepared.

Pieces (2×12 inches, 3×3 inches, or 2.75 cm diameter discs) of UPEmembrane were rinsed three times each with 150 ml of HPLC gradeisopropanol and then three time each with 150 ml of HPLC grade methanol.The membranes were then air dried until they reach a constant weight.The membranes were then pre-wet in methanol and soaked in 50 ml of"NAFION" Solution (sulfonic acid or sodium sulfonate of perfluorinatedpolyether ion exchange powder in lower aliphatic alcohols and 10% water,5 wt. % solution, Aldrich Chemical Co.) for about 5 minutes. The"NAFION" Solution was then decanted and the membranes were bathed inmethylene chloride. The membranes were then rinsed three times each in150 ml of methylene chloride, air dried for 2 hours, and dried undervacuum overnight (15 hours).

The sulfonic acid groups on the membrane were converted to the sulfonylchloride form by reaction with phosphorus pentachloride, analogous toforming an acid chloride from a carboxylic acid as in Example 1, toresult in an activated membrane. Thus, a 2×12 inch, 3×3 inch, or 2.75 cmdiameter piece of "NAFION"-coated UPE membrane was immersed in enoughphosphorus pentachloride solution to completely cover the surface of themembrane. The membrane remained immersed for 8-14 hours to enable thephosphorus pentachloride to convert the sulfonic acid groups to sulfonylchloride groups. This activated membrane containing sulfonyl chloridegroups was then removed from the phosphorus pentachloride solution andwashed thoroughly in hexane or toluene. The activated membrane was thenplaced in a flask containing a solution of 3 g of pentaethylenehexamineligand and enough toluene to ensure complete coverage of the membrane.This mixture was allowed to react for 8-14 hours to form a sulfonamidebond between one of the amine groups of the ligand and a sulfonylchloride group of the activated membrane. The membrane was again washedwith organic solvent to remove unbound ligand and permitted to air dry.

Other ligand derivatized membranes can also be prepared by following theabove guidelines. Also ligands may be attached to sulfonic acidderivatized membranes in the manner described above through theformation of sulfonamide or sulfonate ester bonds.

Metal Ion Recovery and Concentration

The metal ion recovery and concentration process of the inventionrelates to the selective recovery of selected metal ions from mixturesthereof with other metal ions using the compositions of the invention asdefined above. Effective methods of recovery and/or separation of metalions from culinary water supplies, high purity fluids, waste solutions,deposits and industrial solutions and metal recovery from wastesolutions, e.g., from emulsions on photographic and X-ray films,represent a real need in modern technology. These ions are typicallypresent at low concentrations in solutions containing other ions at muchgreater concentrations. Rence, there is a real need for a process toselectively recover and concentrate these undesirable hazardous and/ordesirable ions. The present invention accomplishes this separationeffectively and efficiently by the use of ligands bonded to membranes inaccordance with the present invention.

The general method for selectively recovering and concentrating metalions from solutions of mixed ions involves complexing selected ions in asource solution with a composition of the present invention and thenbreaking the complex to liberate the complexed ions and dissolving theliberated ions in a receiving liquid in a much smaller volume than thevolume of the source solution. As used herein, "source solution,""loading solution," and the like means a solution containing a mixtureof an ion or ions that are selected to be concentrated, separated,and/or recovered together with other ions and complexing or chemicalagents that are not selected to be removed but which are present in muchgreater concentrations in the solution. As used herein, "receivingsolution," "stripping solution," "elution solution," "eluant," and thelike means an aqueous solution that has greater affinity for the ionsthat are to be concentrated, separated, and/or recovered, or in whichsuch ions are soluble. In either event, the selected ions arequantitatively stripped from the ligand in concentrated form in thereceiving solution, because the receiving solution will ordinarily havea much smaller volume than the source solution.

The method of using the membrane/ligand compositions of the presentinvention for separating selected ions from solutions will be apparentto those skilled in the art upon examination of the followingillustrative examples.

EXAMPLE 14

A 0.2 g sheet of the membrane of Example 1 was placed in a beakercontaining 25 ml of 5×10⁻⁴ M CuCl₂ in 1 M sodium acetate and 0.1 Macetic acid (pH=5.5). The membrane was contacted with this sourcesolution for 120 minutes. The membrane was then removed from the sourcesolution, rinsed with water, and placed in 5 ml of receiving solutionconsisting of 1 M HCl.

The source and receiving solutions were analyzed before and aftercontact with the membrane for copper and sodium using flame atomicabsorption (AA) spectroscopy. Initially, the source solution contained23 g/l sodium and 31 ppm copper, but after contact with the membrane itcontained 23 g/l sodium and about 1 ppm copper.

The receiving solution initially contained copper and sodium levelsbelow the level of detection, but after contact with the membranecontained an undetectable amount of sodium and 154 ppm copper. Thisexample shows that the membrane-ligand separation was highly selectivefor copper over sodium, that copper was readily removed from the sourcesolution by contact with the membrane, and that the copper ions could berecovered in a small volume of receiving solution. It is expected thatconcentration of copper ions in the receiving solution would be evengreater when larger volumes of source solution and larger membranes areused.

EXAMPLE 15

A 0.2 g sheet of the membrane of Example 3 was placed in a beakercontaining 25 ml of 5×10⁻⁴ M Hg(NO₃)₂, 0.1 M Ca(NO₃)₂, and 0.5 M NaNO₃.The membrane was contacted with this source solution for 120 minutes.The membrane was then removed from the source solution, rinsed withwater, and placed in 5 ml of a receiving solution consisting of 0.5 Mthiourea, 0.1 M HNO₃.

The source and receiving solutions were analyzed before and aftercontact with the membrane for the presence of mercury using inductivelycoupled plasma (ICP) spectroscopy and for the presence of calcium andsodium using flame atomic absorption (AA) spectroscopy. Initially, thesource solution contained 4 g/l calcium, 12.5 g/l sodium, and 101 ppmmercury. After contact with the membrane, the source solution contained4 g/l calcium, 12.5 g/l sodium, and <1 ppm mercury.

The receiving solution initially contained calcium, sodium, and mercurylevels below the level of detection. After contact with the membrane,this solution contained calcium and sodium at levels below the level ofdetection and mercury at 505 ppm. Thus, mercury was separated from thesource solution also containing sodium and calcium with a high degree ofselectivity. The mercury was readily removed from the source solutioncontaining a mixture of ions, and the mercury was recovered andconcentrated by elution in a simple receiving solution. As with Example14, it is expected that the concentration factor can be improved with asystem operating on a larger scale, particularly with the membraneengineered in cartridge form.

EXAMPLE 16

A 0.2 g sheet of the membrane of Example 4 was placed in a beakercontaining 25 ml of 5×10⁻⁴ M AgNO₃, 0.1 M Fe(NO₃)₃, and 0.1 M NaNO₃. Themembrane was contacted with this source solution for 120 minutes. Themembrane was then removed from the source solution, rinsed with water,and placed in 5 ml of a receiving solution consisting of 6 M HCl.

The source and receiving solutions were analyzed before and aftercontact with the membrane for the presence of silver, iron, and sodiumusing, flame AA spectroscopy. Initially, the source solution contained5.6 g/l iron, 12.5 g/l sodium, and 54 ppm silver. After contact with themembrane, the source solution contained 5.6 g/l iron, 12.5 g/l sodium,and <1 ppm silver.

The receiving solution initially contained iron, sodium, and silverlevels below the level of detection. After contact with the membrane,however, the receiving solution contained undetectable levels of ironand sodium and 265 ppm silver. The membrane-ligand combination washighly selective for removing silver ions from a source solution ofmixed ions. The silver ions thus could be recovered and concentrated inpurified form.

EXAMPLE 17

A 0.2 g sheet of the membrane of Example 5 was placed in a beakercontaining 25 ml of 5×10⁻⁴ M PdCl₂ in 6 M HCl, 0.1 M NiCl₂, 0.1 M FeCl₃,and 0.1 M ZnCl₂. The membrane was contacted with this source solutionfor 120 minutes. The membrane was then removed from the source solution,rinsed in water, and placed in 5 ml of a receiving solution consistingof 2 M NH₃ and 1 M HCl.

The source and receiving solutions were analyzed before and aftercontact with the membrane for palladium, nickel, and zinc using ICPspectroscopy. Initially, the source solution contained 5.9 g/l nickel,5.6 g/l iron, 6.5 g/l zinc, and 52 ppm palladium. After contact with themembrane, the source solution contained 5.9 g/l nickel, 5.6 g/l iron,6.5 g/l zinc, and <1 ppm palladium.

The receiving solution initially contained nickel, iron, zinc, andpalladium at levels below the level of detection. After contact with themembrane, however, the receiving solution contained undetectable levelsof nickel, iron, and zinc, but contained 262 ppm palladium. Thus, themembrane-ligand combination was highly selective for binding palladiumions from a source solution containing a mixture of ions, and permittedremoval, purification, and recovery of the palladium ions.

EXAMPLE 18

A 0.2 g sheet of the membrane of Example 6 was placed in a beakercontaining 25 ml of 5×10⁻⁴ M Pb(NO₃)₂ in 1 M HNO₃, 0.1 M Mg(NO₃)₂, and0.1 M Ca(NO₃)₂. The membrane was contacted with this source solution for120 minutes. The membrane was then removed from the source solution,rinsed with water, and placed in 5 ml of a receiving solution consistingof 0.03 M tetrasodium EDTA.

The source and receiving solutions were analyzed before and aftercontact with the membrane for the presence of lead, magnesium, andcalcium using flame AA spectroscopy. Initially, the source solutioncontained 2.4 g/l magnesium, 4.0 g/l calcium, and 102 ppm lead. Aftercontact with the membrane, the source solution contained 2.4 g/lmagnesium, 4.0 g/l calcium, and about 2 ppm lead.

The receiving solution initially contained magnesium, calcium, and leadat levels below the level of detection. After contact with the membrane,the receiving solution contained undetectable levels of magnesium andcalcium and 495 ppm lead. Thus, the membrane-ligand combination washighly selective in removing lead ions from a source solution containinga mixture of ions and permitted recovery and concentration of relativelypure lead.

EXAMPLE 19

A 0.2 g sheet of the membrane of Example 7 was placed in a beakercontaining 25 ml of 5×10⁻⁴ M Sb in 2 M H₂ SO₄, 0.3 M CuSO₄, and 0.1 MNiSO₄. The membrane was contacted with this source solution for 120minutes. The membrane was then removed from the source solution, rinsedin water, and placed in 5 ml of a receiving solution consisting of 6 MHCl.

The source and receiving solutions were analyzed before and aftercontact with the membrane for copper, nickel, and antimony using flameAA spectroscopy. Initially, the source solution contained 5.9 g/lnickel, 19 g/l copper, and 56 ppm antimony. After contact with themembrane, the source solution contained 5.9 g/l nickel, 19 g/l copper,and <5 ppm antimony.

The receiving solution initially contained nickel, copper, and antimonyat levels below the level of detection. After contact with the membrane,however, the receiving solution contained undetectable levels of nickeland copper, but contained 285 ppm antimony. Thus, the membrane-ligandcombination was selective for binding antimony from a source solutioncontaining a mixture of ions, and permitted removal, purification, andrecovery of the antimony.

EXAMPLE 20

A 0.2 g sheet of the membrane of Example 8 was placed in a beakercontaining 25 ml of 5 ppm iron, 5 ppm lead, 5 ppm copper, 5 ppm nickel,and 5 ppm zinc in tap water. Tap water contains relatively highconcentrations of sodium, potassium, calcium, and magnesium ions. Themembrane was contacted with this source solution for 240 minutes. Themembrane was then removed from the source solution, rinsed in water, andplaced in 5 ml of a receiving solution consisting of 6 M HCl.

The source and receiving solutions were analyzed before and aftercontact with the membrane for iron, nickel, and zinc using ICPspectroscopy and for copper and lead using flame AA spectroscopy.Initially, the source solution contained the levels of each metal asmentioned above. After contact with the membrane, the source solutioncontained <1 ppm of each of the metals.

The receiving solution initially contained iron, lead, nickel, copper,and zinc at levels below the level of detection. After contact with themembrane, however, the receiving solution contained 25 ppm nickel, 25ppm copper, 24 ppm iron, 26 ppm lead, and 26 ppm zinc. Thus, themembrane-ligand combination readily removed iron, lead, copper, nickel,and zinc from a source solution containing a mixture of ions despite thepresence of sodium, potassium, calcium, and magnesium ions in the sourcesolution.

EXAMPLE 21

A 0.2 g sheet of the membrane of Example 9 was placed in a beakercontaining 25 ml of 200 ppb iron in 1% HF. The membrane was contactedwith this source solution for 480 minutes. The membrane was then removedfrom the source solution, rinsed in water, and placed in 5 ml of areceiving solution consisting of 37% HCl.

The source and receiving solutions were analyzed before and aftercontact with the membrane for iron using graphite furnace AAspectroscopy. Initially, the source solution contained 200 ppb iron.After contact with the membrane, the source solution contained 10 ppbiron.

The receiving solution initially contained iron at a level below thelevel of detection. After contact with the membrane, however, thereceiving solution contained 910 ppb iron. Thus, the membrane-ligandcombination readily removed iron from the source solution despite thevery low level of iron in the source solution and the presence of bothacid and the strongly iron-chelating fluoride.

EXAMPLE 22

A 0.2 g sheet of the membrane of Example 10 was placed in a beakercontaining 25 ml of 10 ppm iron, 10 ppm copper, and 10 ppm nickel in 0.5M HF and 0.5 M NaF. The membrane was contacted is with this sourcesolution for 240 minutes. The membrane was then removed from the sourcesolution, rinsed in water, and placed in 5 ml of a receiving solutionconsisting of 3 M HCl.

The source and receiving solutions were analyzed before and aftercontact with the membrane for iron and nickel using ICP spectroscopy andfor copper using flame AA spectroscopy. Initially, the source solutioncontained 10 ppm each of iron, copper, and nickel. After contact withthe membrane, the source solution contained <1 ppm of each of the threemetals.

The receiving solution initially contained iron, copper, nickel, andsodium at levels below the level of detection. After contact with themembrane, however, the receiving solution contained sodium at a levelbelow the level of detection and 50 ppm each of iron, copper, andnickel. Thus, the membrane-ligand combination readily removed iron,copper, and nickel from the source solution, and these three metalscould be separated from the source solution and recovered.

EXAMPLE 23

A 0.2 g sheet of the membrane of example 11 was placed in a beakercontaining 25 ml of 5 ppm potassium in deionized distilled water at pH8. The membrane was contacted with this source solution for 120 minutes.The membrane was then removed from the source solution, rinsed in water,and placed in 5 ml of a receiving solution consisting of 0.1 M HCl.

The source and receiving solutions were analyzed before and aftercontact with the membrane for potassium using flame AA spectroscopy.Initially, the source solution contained 15 ppm potassium, but aftercontact with the membrane it contained <1 ppm of potassium.

The receiving solution contained potassium at a level below the level ofdetection, but after contact with the membrane contained 75 ppmpotassium. Thus, potassium could be readily removed from the sourcesolution by binding to the membrane and recovered by elution in areceiving solution.

EXAMPLE 24

A 0.2 g sheet of the membrane of example 12 was placed in a beakercontaining 25 ml of 5 ppm of each of lead, cadmium, mercury, copper, andnickel in tap water. The membrane was contacted with this sourcesolution for 480 minutes. The membrane was then removed from the sourcesolution, rinsed with water, and placed in 5 ml of a receiving solutionconsisting of 6 M HCl.

The source and receiving solutions were analyzed before and aftercontact with the membrane for mercury, cadmium, and nickel using ICPspectroscopy, and for lead, copper and mercury using flame AAspectroscopy. Initially, the source solution contained 5 ppm of each oflead, cadmium, mercury, copper, and nickel, but after contact with themembrane it contained <1 ppm of each of these elements.

The receiving solution initially contained lead, cadmium, mercury,copper, and nickel at levels below the level of detection. After contactwith the membrane, however, the receiving solution contained 25 ppm ofeach of the elements. Hence, lead, cadmium, mercury, copper, and nickelwere all readily removed from a source solution also containing sodium,potassium, calcium, and magnesium. Further, all of the elements removedfrom the solution by adsorption to the membrane were recovered andconcentrated in the receiving solution.

EXAMPLE 25

A 0.04 g sheet (2.75 cm diameter disc) of the membrane of Example 13 wasplaced in a membrane holder (O-ring and clamp). This arrangement allowedfor a 1.83 cm diameter portion of the disc to be in contact with asolution flowing through the membrane. A 5 ml source solution containing6 ppm Cu in 1 M Zn(NO₃)₂, 0.1 M sodium acetate, and 0.01 M acetic acidwas passed through the membrane using vacuum suction from a vacuum pumpat a flow rate of 1 ml/min. The membrane was then washed by flowing 2 mlof 1 M NH₄ Cl through the membrane at 1 ml/min. Next, 3 ml of areceiving solution comprising 0.5 M HCl was passed through the membraneat a flow rate of 2 ml/min.

The source and receiving solutions were analyzed before and after theywere passed through the membrane for copper, zinc, and is sodium usingflame atomic absorption spectroscopy. Initially, the source solutioncontained 6 ppm Cu, 65 g/l Zn, and 2.3 g/l Na. After contact with themembrane, the Zn and Na levels in the source solution were unchanged,and the Cu level was 2 ppm.

The receiving solution initially contained Cu, Zn, and Na at levelsbelow the level of detection. After passing through the membrane,however, the receiving solution contained undetectable levels of Zn andNa, but contained 10 ppm Cu. Thus, the membrane-ligand combination washighly selective for the Cu at low levels in a source solutioncontaining concentrated Zn and Na.

From the foregoing, it will be appreciated that the ligand-membranecompositions of the present invention provide a material useful forseparation, recovery, and concentration of selected metal ions frommixtures of those ions with other ions, even when those other ions arein far greater concentrations. The recovered metals can then be analyzedor further concentrated from the receiving solution by standardtechniques known in the technology of these materials.

Although the process of separating and concentrating certain metal ionsin this invention has been described and illustrated by reference tocertain specific membrane-bound ligands, processes using analogs ofthese ligands are within the scope of the processes of the invention asdefined in the following claims.

We claim:
 1. A method of making a composition suitable for selectivelyremoving ions from solutions comprising the step of reacting an ionbinding ligand with a composite membrane to form a covalent bond linkingthe ion-binding ligand to the composite membrane, said compositemembrane comprising a porous substrate which is coated with across-linked hydrophilic polymer.
 2. The method of claim 1, wherein theion-binding ligand includes a functional grouping and the membraneincludes an activated polar group, wherein the functional grouping isreactive with the activated polar group.
 3. The method of claim 2,wherein the covalent bond linking the ion-binding ligand to the membraneis selected from the group consisting of amide (NHCO), ester (COO),thioester (COS), carbonyl (CO), ether (O), thioether (S), sulfonate(SO₃), and sulfonamide (SO₂ NH).
 4. The method of claim 2, wherein thecomposite membrane is formed by a method comprising the steps of:coatinga porous substrate with a monomer; and polymerizing and cross-linkingthe monomer, wherein the polymerized and cross-linked monomer ishydrophilic and includes ligand-reactive functional groups.
 5. Themethod of claim 4, wherein the monomer is polymerized by free radicalpolymerization.
 6. The method of claim 5, wherein the porous substrateincludes pores which are also coated with the monomer.
 7. The method ofclaim 6, wherein the size of the pores is between about 0.001 and 10micrometers.
 8. The method of claim 6, wherein the size of the pores isbetween about 0.1 and 5.0 micrometers.
 9. The method of claim 2, whereinthe composite membrane is formed by a method comprising the stepsof:washing the porous substrate with a solvent prior to coating thesubstrate with the monomer, thereby wetting the surface of the membrane;and bathing the substrate in a mixture including the monomer, apolymerization initiator, and a cross-linking agent under conditionseffective for free radical polymerization and cross-linking of themonomer and coating of the substrate with the cross-linked polymer,thereby forming the composite membrane.
 10. The method of claim 9,wherein the composite membrane includes a surface that includes a moietyselected from the group consisting of carboxylic acid, hydroxyl,sulfonic acid, epoxy, primary amine, and derivatized benzyl groups. 11.The method of claim 2, wherein the composite membrane is formed bymethod comprising the steps of:washing a porous substrate with asolvent, thereby wetting the surface of the substrate; bathing theporous substrate in a solution including crystals; removing the poroussubstrate from the solution, to produce a porous substrate havingcrystals adhered thereto; treating the porous substrate with a compoundthat precipitates and fixes the crystals adhered to the poroussubstrate, thereby producing the composite membrane.
 12. The method ofclaim 2, wherein the composite membrane comprises a carboxylic acidfunctional group that reacts with the ligand to form the covalent bond.13. The method of claim 12, wherein the ligand comprises a pendant aminegroup, the carboxylic acid functional group reacting with the pendantamine group to form an amide bond.
 14. The method of claim 2, whereinthe composite membrane comprises a sulfonic acid functional group thatreacts with the ligand to form the covalent bond.